1. Field of the Invention
The present invention relates to an amplifier including a device for correcting or adjusting the output offset voltage of either the amplifier, or a processing circuit including the amplifier. The invention more particularly relates to an amplifier, such as a magnetic head amplifier, having an active state in which its output is taken into account by further circuits and an inactive state in which its output is ignored.
2. Discussion of the Related Art
When an AC signal must be processed (amplified, filtered, etc.), it is often necessary to eliminate, at one or several stages of the processing circuit, DC components which, if they become too high after successive amplifications, could cause a clipping of the processed signal. The DC components, or offset voltages, are particularly impairing in some applications, such as processing of signals provided by magnetic heads, in which they are very high compared to the effective AC component of the processed signal.
To eliminate a DC component, a capacitor is usually inserted in the path of the processed signal. However, if it is desired to pass low frequencies, as for example in sound processing, the values of the capacitors must be particularly high, which does not allow integration of the capacitors into a circuit. Furthermore, in a processing chain, it is often necessary to eliminate the DC components at several stages and therefore to insert a plurality of capacitors, thus requiring the provision of the same number of additional pins in the integrated circuit when the processing circuit is integrated.
FIG. 1 represents a conventional circuit using a single capacitor to eliminate the DC component, or to adjust the component at a desired value at the output of a processing chain, independently of the offset voltages added to the signal upstream.
In the example of FIG. 1, the processing chain includes an operational amplifier 10 followed by a filter 12 providing the output signal Vout of the processing chain. The elements of the circuit are supplied between a high voltage V+ and a low voltage V-, for example ground. The input signal Vin of the chain is provided at the noninverting input of amplifier 10. Amplifier 10 includes a feedback loop including a resistor R1 connected between its output and its inverting input, and a resistor R2 connected between the inverting input and a correction or compensation voltage Vc. The correction voltage Vc is provided through an integrator 14 which integrates the difference between the output voltage Vout and a reference voltage Vref.
In this configuration, the DC component of the output voltage Vout establishes at value Vref, independently of the elements found upstream the output Vout. Preferably, voltage Vout is selected at the average of the supply voltages V+ and V-, for example by a bridge of resistors R3 and R4 of equal value. Thus, the output voltage can have the same amplitude on both sides of its DC component Vref. An integrator 14 requires an integration capacitor CI that is generally connected between the output and the inverting input of an operational amplifier. If the processed signal is liable to have a low frequency (for example 10 Hz in sound processing), capacitor CI must have a high value to prevent the DC component of signal Vout from oscillating with the low frequency. Therefore, capacitor CI may be physically too large to be integrable, and thus two additional connection pins must be provided on a circuit integrating the processing circuits to connect to two terminals of an external capacitor CI.